CPR, as manually applied by human rescuers, is generally a combination of techniques including artificial respiration (through rescue breathing, for example) and artificial circulation (by chest compression). One purpose of CPR is to provide oxygenated blood through the body, and to the brain, in those patients where a prolonged loss of circulation places the patient at risk. For example after a period of time without restored circulation, typically within four to six minutes, cells in the human brain can begin to be damaged by lack of oxygen. CPR techniques attempt to provide some circulation, and in many cases, respiration, until further medical treatment can be delivered. CPR is frequently, though not exclusively, performed on patients who have suffered some type of sudden cardiac arrest such as ventricular fibrillation where the patient's natural heart rhythm is interrupted.
It has been found that the desired effects of CPR, when delivered manually, can suffer from inadequate performance. In order to have the greatest chance at success, CPR must typically be performed with some degree of force for an extended period of time. Often the time and exertion required for good performance of CPR is such that the human responder begins to fatigue. Consequently the quality of CPR performance by human responders may trail off as more time elapses. Mechanical CPR devices have been developed which provide chest compression using various mechanical means such as for example, reciprocating thrusters, or belts or vests which tighten or constrict around the chest area. In these automated CPR devices, motive power is supplied by a source other than human effort such as, for example, electrical power or a compressed gas source. Mechanical CPR devices have the singular advantage of not fatiguing as do human responders. Additionally, mechanical CPR devices may be advantageous when no person trained or qualified in manual CPR is able to respond to the patient. Thus, the advent of mechanical CPR devices now allows for the consistent application of CPR chest compressions for extended periods of time.
When a patient experiences cardiac arrest, the heart ceases to pump blood throughout the body. The cessation of blood flow is known as ischemia. When CPR chest compressions are commenced, some blood flow is restored. The restoration of blood flow after a period of ischemia is known as reperfusion. The study of CPR has revealed that after initial resuscitation from cardiac arrest, a cardiovascular postresuscitation “syndrome” often ensues, characterized by various forms of cardiac dysfunction. In many cases, this postresuscitation dysfunction can lead to heart failure and death. Furthermore, the study of reperfusion after ischemia has revealed that a particular kind of injury can develop in the first moments of reperfusion. This injury, known as ischemia/reperfusion injury, occurs for reasons not fully understood. It, however, is known to result in a variety of symptoms that can contribute to postresuscitation cardiac dysfunction. More importantly, ischemia/reperfusion injury is known to be affected by the quality of reperfusion experienced after a period of interrupted blood flow. A cardiac arrest patient, who has had no blood flow for several minutes, and who then receives CPR for some period of time, may be expected to experience ischemia/reperfusion injury.
Without wishing to be bound by any theory, the following explanation is offered to illustrate the current understanding of ischemia/reperfusion injury. Generally, ischemia/reperfusion injury initiates at the cellular level and chemically relates most strongly to the transition between conditions of anoxia/hypoxia (insufficient oxygen) and ischemia (insufficient blood flow), and conditions of proper oxygenation and blood flow. Pathophysiologically, reperfusion is associated with a variety of deleterious events, including substantial and rapid increases in oxidant stress, intracellular calcium accumulation, and immune system activation. These events can spawn a variety of injury cascades with consequences such as cardiac contractile protein dysfunction, systemic inflammatory response hyperactivation, and tissue death via necrosis and apoptosis. Unfortunately, following cardiac arrest, ischemia/reperfusion injury and the resulting postresuscitation “syndrome” is serious enough to cause recovery complication and death in many instances.
Hence, there exists a need for an improved mechanical CPR device and methods for using the same. It would be desired to develop CPR methods, and particularly CPR methods for use with a mechanical CPR device, that lessen the severity of ischemia/reperfusion injury and that offer an improved level of response and patient treatment. The present invention addresses one or more of these needs.